Polyester molding compound is used in the manufacture of light-weight, glass reinforcedpolyester resin automotive parts and other general hardware which is made in heated matched metal dies or molds. Polyester molding compound is conventionally produced in the form of sheet molding compound (SMC) or bulk molding compound (BMC).
Sheet molding compound is made by dropping glass fibers onto the surface of a polyethylene film which has first been coated with a non-polymerized polyester resin paste. A similarly coated polyethylene film is placed coating to coating over the first film to form a sandwich composite. This composite is then squeezed to remove excess air and taken up onto a spool. This spool is stored at 70.degree. -90.degree. F. for one to ten days.
During this curing period, thickeners in the resin paste increase the paste viscosity from that of a relatively low viscosity liquid until ultimately a dry leather-like molding compound containing long glass fibers randomly dispersed in two dimensions is produced. In this manner fiber integrity is largely maintained since no intensive mixing is involved in the process. Fiber integrity is important in as much as the strength of the fiber reinforced composite is a function of fiber length, its dispersion, and its loading in the resin.
In order to obtain consistent manufacturing conditions, the initial paste viscosity of the polyester resin paste must be held within certain defined limits. The thickener in the resin is designed such that the viscosity of the paste increases slowly until after the glass fibers have had a chance to become coated or "wet out" in the resin. After a sufficient time has been allowed for thorough glass wet-out, the thickeners present in the paste affect a rapid increase in viscosity until approximately 20-60 million-centipoise (MMcps) is attained. Thereafter, the rate of increase in viscosity slows down. Accordingly, the initial increase in viscosity must be slow and reproducible in order to facilitate the manufacture of sheet molding compound in a highly automated environment
Bulk molding compounds are similar in chemical composition to SMC compounds. The manufacturing process differs in that low intensity mixers are used to gently wet-out the glass fibers into the resin paste. This process allows the use of higher viscosity pastes. Thickeners may be used to obtain a further viscosity increase in some situations. While mixing times are kept as short as possible to minimize fiber degradation, extensive degradation does occur with fibers longer than 1/4" in length. Thus, the BMC process typically employs fibers of this length or smaller.
Calcium carbonate fillers are used at high levels in polyester molding compounds to reduce costs, to improve surface finish by reducing resin shrinkage, and to modify the rheological behaviors of the paste to prevent segregation of the fiberglass during handling and storage. Although fillers are typically viewed as low cost bulking agents, they are critical to the processing of polyester molding compounds. The physical properties of the fillers must be maintained within certain tightly controlled limits to produce composites of consistent quality.
As the filler levels have increased through the years to provide improved surface finish, it has been a disadvantage that minor variations in the chemical and physical properties of the filler can cause significant variations in viscosity profile. This in turn results in manufacturing difficulties when using highly automated equipment. Further, variations in filler moisture levels are especially serious in their effect on the thickening rate of the polyester resin paste.
Because of its inherently low oil absorption, low moisture content and low cost, the most preferred filler used in polyester molding compound applications is calcium carbonate. The ores from which finely ground calcium carbonate fillers are produced for use in polyester molding compounds are found naturally in several forms. Broadly speaking, these forms are chalk, limestone, marble and dolomite. Precipitated calcium carbonate, which is a synthetically prepared product, is unacceptable for polyester molding compound applications due to its excessively high oil absorption.
Chalk is a soft, amorphous carbonate made up of the fossil shells of millions of tiny marine organisms. Accordingly, while the individual particles are rounded and quite strong in themselves, the bonding between particles is weak and easily broken. Hence, the comminution of chalk is essentially a process of gently breaking down the mineral into its fundamental particles. This may be accomplished either by dry grinding the chalk or wet grinding the chalk in a water slurry and segregating the desired fraction using water flotation techniques. Historically, the term "whiting" was used to describe finely pulverized chalk. This term is now inconsistently used to describe any finely ground calcium carbonate from any source, and therefore, for clarity this term is best avoided.
Limestone, marble, and dolomite are generally crystalline rocks whose particles and grinding display the characteristic rhombohedral structure of compact but pointed particles. Marble, limestone and dolomite are conventionally processed by one of two methods. The first and most energy efficient is dry grinding. Dry-ground limestone has been found unacceptable for use in polyester molding compounds because of excessive resin demand and excessive variation in the rate of viscosity increase in the manufacturing process.
The second method for processing limestone or marble is wet grinding wherein the limestone is crushed, made into a slurry with water and ground to the desired particle size. Thereafter, the limestone is filtered from the slurry and dried to produce a calcium carbonate filler. Wet ground calcium carbonates are the preferred filler in polyester molding compounds.
The wet grinding process requires the removal of substantial quantities of water from the processed slurry and, consequently, .this process requires substantially more energy than the dry grinding method. It is generally accepted that wet grinding calcium carbonate is between 15% to 25% more expensive than the dry grinding technique. Accordingly, it has long been desired to develop a dry-ground calcium carbonate which has the thickening consistency and loading capability of wet ground products Although attempts have been made to surface treat many of the above calcium carbonate fillers with silanes or fatty acids, such as stearic acid, these treatments have not been entirely successful although dispersion and oil absorption have been improved.
In the manufacture of polyester molding compound, two grades or types of calcium carbonate filler are used. The first is termed a Class I filler. Class I calcium carbonate fillers are a finely ground material having a median particle size between 2.0 to 4.0 microns and with at least 90% by weight less than 10 microns. This fine particle size filler is used in applications where surface finish is the most critical. However, because of the fine particle size, filler loadings must necessarily be restricted to lower levels. This is because of the higher oil absorption and resin demand of smaller particle size fillers. Class I fillers are known to produce composites having higher strength and supcrior surface finish.
The second type of filler is termed a Class II product. A Class II filler has a median particle size between approximately 5 to 10 microns with at least 98% by weight finer than 44 microns. A Class II filler has a lower surface area than the Class I filler and hence has a lower oil absorption and resin demand. Accordingly, Class II fillers may be placed in polyester molding compounds at much higher loading levels. The fraction greater than 44 microns must be kept below two percent to minimize imperfections in the composite surface. Therefore, samples made according to the present invention were formed into fillers of both the Class I and Class II variety for comparison with other fillers of the same class.
While calcium carbonate deposits are widely dispersed throughout the world, only a very few meet the exacting requirements necessary for a polyester molding compound filler. In the United States only high purity marble fillers have gained any significant usage. In Europe the high purity limestone from the Orgon deposit in France is preferred. Chalks, despite their wide spread availability, their ease of processing, and their high calcium carbonate content have not found extensive application as polyester molding compound fillers at the more preferred high loadings because of their extremely high oil absorption and moisture pickup values. Both of these factors follow from their extreme fineness and porosity.
The polyester molding compound maturation process is extremely moisture sensitive, and accordingly, the moisture content of the filler must be kept below 0.1%. This extreme moisture sensitivity leads to process stability problems. These process stability problems are especially severe during seasonal variations in temperature and relative humidity Because the filler moisture content is a function of the relative humidity of the filler storage area which is substantially uncontrollable, compounders have attempted to overcome these process consistency problems by numerous methods.
Compounders tried developing special seasonal recipes, purchasing resins to which free water has been added to a constant level or limiting the filler loading such that they obtain satisfactory glass wet-out and thickening even under the most adverse relative humidities encountered. These are less than optimum solutions since relative humidities only roughly follow seasonal patterns and limiting filler levels to cover worst case conditions unnecessarily increase the cost of the final resin-aglass composite.
These and other disadvantages of the prior art are overcome by the product and process of the present invention which is set forth in the following detailed description.